The field of the present invention relates to optical devices incorporating distributed optical structures. In particular, apparatus and methods are described herein for implementing amplitude and phase control in distributed diffractive optical structures.
Distributed optical structures in one-, two-, or three-dimensional geometries offer powerful optical functionality and enable entirely new families of devices for use in a variety of areas including optical communications, spectral sensing, optical waveform coding, optical waveform processing, and optical waveform recognition. It is important in the design of distributed optical structures to have means to control the amplitude and phase of the electromagnetic field diffracted by individual diffractive elements within the overall distributed structure. This invention relates to approaches for fabricating diffractive elements that provide flexible control over diffractive amplitude and phase.
A distributed optical structure typically includes a large number of individual diffractive elements. Each individual diffractive element may scatter (and/or reflect and/or diffract) only a small portion of the total light incident on the distributed structure. This may be because the individual diffractive elements subtend only a small fraction of available solid angle of the incident optical field in the interaction region, and/or because individual diffractive elements have a small reflection, diffraction, or scattering coefficient. Distributed optical structures in two or three dimensions can also be described as volume holograms since they have the capability to transform the spatial and spectral properties of input beams to desired forms.
There are many reasons why it is important to have control over the amplitude and/or phase of the portions of the field scattered by individual diffractive elements. For example, a distributed optical structure can act as a general spectral filter supporting a broad range of transfer functions. In the weak-reflection approximation, the spectral transfer function of a structure is approximately proportional to the spatial Fourier transform of the structure""s complex-valued scattering coefficientxe2x80x94as determined by the amplitude and phase of the field scattered by individual diffractive elements (See: T. W. Mossberg, Optics Letters Vol. 26, p. 414 (2001); T. W. Mossberg, SPIE International Technology Group Newsletter, Vol. 12, No. 2 (November 2001); and the applications cited hereinabove). In order to produce a general spectral transfer function, it is useful to control the amplitude and phase of each constituent diffractive element. Application of the present invention provides for such control. Also, when multiple distributed structures are overlaid in the same spatial region, system linearity can only be maintained by ensuring that the diffractive strength of overlaid diffractive elements is the sum of the individual diffractive element strengths. When diffractive elements are lithographically scribed, overlaid structures will not typically produce a summed response. The approaches of the present invention provide means for modifying overlaid diffractive elements (formed by lithographic and/or other suitable means) so that each element negligibly affects another""s transfer function.
An optical apparatus according to the present invention comprises an optical element having a set of multiple diffractive elements. Each diffractive element diffracts a corresponding diffracted component of an incident optical field with a corresponding diffractive element transfer function. Collectively, the diffractive elements provide an overall transfer function between an entrance optical port and an exit optical port (which may be defined structurally and/or functionally). Each diffractive element includes at least one diffracting region modified or altered in some way so as to diffract, reflect, and/or scatter a portion of an incident optical field, and is spatially defined relative to a corresponding one of a set of diffractive element virtual contours. The virtual contours are spatially arranged so that, if the diffracting regions of the corresponding diffractive elements were to spatially coincide with the virtual contours, the resulting superposition of corresponding diffracted components at a design wavelength would exhibit maximal constructive interference at the exit port. The modification to form a diffracting region typically involves a differential between some optical property of the diffracting region relative to the corresponding average optical property of the optical element (effective index, bulk index, surface profile, and so forth).
The optical element may be a planar (2D) or channel (1D) waveguide, with optical field propagation substantially confined in at least one transverse dimension. In a waveguide, the diffracting regions are curvilinear segments having some alteration of an optical property relative to the waveguide. The optical element may be a 3D optical element enabling three-dimensional propagation of optical fields therein, with the diffracting regions being surface areal segments of surface contours within the volume of the optical element. The optical element may be a diffraction grating, the diffracting regions being segments of the grating lines or curvilinear grooves that are formed on the grating. These various distributed optical devices may define one or more ports, and may provide one or more spatial/spectral transfer functions between the one or more ports.
For a channel or planar waveguide, a 3D optical element, or a diffraction grating, the overall transfer function and/or at least one corresponding diffractive element transfer function is determined at least in part by longitudinal and/or angular displacement of at least one diffracting region relative to the corresponding virtual contour. For a planar waveguide, a 3D optical element, or a diffraction grating, the overall transfer function and/or at least one corresponding diffractive element transfer function is determined at least in part by: longitudinal and/or angular displacement of at least one diffracting region relative to the corresponding virtual contour; longitudinal displacement of at least one diffractive element relative to the corresponding virtual contour; and/or at least one virtual contour lacking a diffractive element corresponding thereto.
Various objects and advantages pertaining to distributed optical structures may become apparent upon referring to the preferred and alternative embodiments of the present invention as illustrated in the drawings and described in the following written description and/or claims.